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Portage Lake Volcanics, Keweenaw Co., Michigan, USA

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The Portage Lake Volcanics (PLV) are the host rock unit for both native and sulfide copper deposits. The total thickness of the PLV is about 10 kilometers, of which 5 kilometers are exposed on the Keweenaw Peninsula (Cannon, 1992). The Portage Lake Volcanics consist of over 200 subaerial flood basalt lava flows with occasional interflow sediment layers (Bornhorst, 1997). A typical lava flow is about 10 to 20 meters thick and consists of a massive interior and the uppermost 5 to 20% being a vesicular flow top. About one-fourth of the individual flow tops are brecciated (White, 1968). Interflow sandstones and conglomerates, with a thickness of up to 40 meters, make up less than 5 volume percent of the PLV (Merk and Jirsa, 1982). Both the tops of lava flows, especially brecciated zones, and the interflow conglomerates host native copper ore deposits. Massive interiors do not contain copper or secondary minerals except along fractures and faults (Bornhorst, 1997). On the Keweenaw Peninsula, the PLV is overlain by Oronto Group sediments, including the Copper Harbor Conglomerate, Nonesuch Shale, and Freda Sandstone. The base of the PLV is cut by the once normal, graben bounding Keweenaw Fault, now a major high angle reverse fault with several kilometers of displacement (Bornhorst, 1997).

Mineral List



6 entries listed. 5 valid minerals.

The above list contains all mineral locality references listed on mindat.org. This does not claim to be a complete list. If you know of more minerals from this site, please register so you can add to our database. This locality information is for reference purposes only. You should never attempt to visit any sites listed in mindat.org without first ensuring that you have the permission of the land and/or mineral rights holders for access and that you are aware of all safety precautions necessary.

References

Bornhorst, T.J., 1997, Tectonic context of native copper deposits of the North American Midcontinent Rift System, in Ojakangas, R.W., Dickas, A.B., and Green, J.C., eds., Middle Proterozoic to Cambrian Rifting, Central North America: Boulder, Colorado, Geological Society of America Special Paper 312.

Cannon, W.F., 1992, The Midcontinent rift in the Lake Superior region with emphasis on its geodynamic evolution: Tectonophysics, v. 213, p. 41-48.

Merk, G.P., and Jirsa, M.A., 1982, Provenance and tectonic significance of the Keweenawan interflow sedimentary rocks: Geological Society of America Memoir 156, p. 97-105.

White, W.S., 1968, The native copper deposits of northern Michigan, in Ridge, J.D., eds., Ore Deposits of the United States, 1933-1967 (the Graton Sales volume): American Institute of Mining, Metallurgical and Petroleum Engineering, New York, p. 303-325.

 
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